Recently, composite materials using a carbon nanofiber have been attracted much attention. Such composite materials are expected to have improved mechanical strength owing to the incorporation of a carbon nanofiber. However, since the carbon nanofibers mutually have a strong aggregating property, it is considered to be very difficult to homogeneously disperse the carbon nanofibers into substrates of composite materials. Therefore, it is currently difficult to obtain a composite material of a carbon nanofiber having desired properties and also it is impossible to efficiently utilize an expensive carbon nanofiber.